The Crystal Structure of Silicon Carbide

Silicon carbide crystallizes into an interlocked structure which is covalently bonded together. This structure forms different polytypes due to various atoms taking up different sites within its lattice framework.

Hexagonal a-SiC and rhombohedral b-SiC are among the many non-cubic polytypes known; of these only two have any technological significance. Other polytypes have also been documented; only some may prove useful.

Hexagonal a-SiC

Silicon carbide (SiC) is a multifaceted covalent material with multiple crystal structures. Mechanical engineers use SiC in applications like armor; its dynamic response under intense loading conditions has long been studied in order to enhance performance and durability of this material. One key factor influencing dynamic deformation in SiC is the interaction between polytypes and various loading conditions.

SiC is known for having more than 250 variations, commonly referred to as polytypes. Each polytype can be identified by its relative percentage of hexagonal close-packing in its lattice structure and includes six common poly types – 6H, 15R, 4H and wurtzite (2H-SiC). Hexagonal a-SiC contains the highest proportion of hexagonality (33%). Cubic-to-hexagonal transformation (67%) ranks second; most commercially available SiC comes as wurtzite (31%).

All polytypes feature a tetragonal lattice structure with sixhold symmetry, meaning each carbon atom has two neighbors at equal distance from it in the crystal lattice structure, which dictates its position within it and stacking sequence of layers within each polytype – defined by their periodic stacking sequence; this definition also applies to binary tetrahedral materials like zinc blende.

Miller-Bravais indices a1, a2, a3, and c define the crystallographic axes of any polytype crystal structure, representing unambiguously assignable principal vectors for any given crystal structure. Of these four indices, only the Miller-Bravais index a1 stands out as being most often and regularly assigned, as per any crystal structure; its plane being closest packed while perpendicularly it runs perpendicularly into it – its plane also being one of three commonly occurring and stable Miller-Bravais Indices used.

The a-plane is integral to processing SiC wafers and can have significant impacts on their physical properties such as thermal conductivity, electrical conductivity and interface density of states. Furthermore, its existence influences various industrial applications like epitaxial growth, ion implantation and metal deposition – it’s therefore important that one understands this crystal structure to optimize performance of SiC wafers.

Cubic b-SiC

Silicon carbide is a strong chemical compound with strong covalent bonds, boasting wide band-gap semiconductor properties and high melting and boiling points, giving it wide use. Silicon carbide’s wide melting/boiling points allow it to be utilized both as structural ceramic material or doped with nitrogen, phosphorus or beryllium dopants to form n-type and p-type semiconductors for use in applications requiring high temperatures like gas turbines or nuclear reactors.

Edward Goodrich Acheson first successfully mass produced carborundum on a large scale in 1891 while trying to produce artificial diamonds. Heating a mixture of clay (aluminium silicate) and powdered coke in an iron bowl led him to believe he had produced a compound similar to corundum with similar carbon and aluminium compounds; once cool it formed blue crystals that Acheson believed were formed from this new compound; which he named carborundum and later patented it; its popularity continued as a long lasting industrial abrasive until 1929 when boron carbide came along, where after it came back into use as a long lasting ceramic used in automotive brakes and clutches as long lasting ceramic materials found within automotive brakes and clutches as long lasting ceramic materials used as long-term automotive brakes and clutches brake pads or clutches used within automotive brakes and clutches.

SiC has an alternating layer structure stacked in specific sequence. Each layer comprises silicon atoms bonded to carbon atoms, producing a cubic lattice with a tetrahedral bonding geometry and various stacking sequences producing different polytypes with close-packed layers having lattice parameter values equaling those for close-packed layers (120deg between adjacent layers) as well as having 120deg between adjacent tetrahedral angles between layers; all named according to its crystal structure and number of layers that form a unit cell (see table below).

As opposed to cubic metals, SiC’s Miller indices of its three-dimensional structure do not equal each other, yet three indices suffice in unambiguously describing a plane or direction; an a-plane, for instance, can be defined using Miller indices a, b and c as shown below (c-axis being parallel).

Because each polytype has a slightly different stacking sequence, some properties of their crystal aren’t identical across its entirety; this phenomenon is known as anisotropy. Rhombohedral polytypes 15R and 33R possess isotropic crystal structures but exhibit electrical parameter variations depending on crystal orientation; to understand their effects more fully bulk techniques like Glow Discharge Mass Spectrometry and X-Ray Fluorescence Spectrometry must also be employed along with Inductively Coupled Plasma-Optical Emission Spectrometry/ICP-Mass Spectrometry measurements when digested or leached samples are analysed.

Rhombohedral a-SiC

Rhombohedral SiC crystal structure is an igneous rock-derived form of layered silicate that has long been utilized as an abrasive and gemstone material. Due to its unique crystal structure, rhombohedral SiC has long been utilized as cutting tools or other mechanical components; naturally found in diorite, quartz, schists as well as metamorphic rocks like gneiss and amphibole as natural crystalline forms as well as polycrystalline and polymorphic forms as industrial forms.

Silicon carbide stands out among semiconductors as having multiple polytypes with distinct bonding arrangements that can be distinguished through stacking sequences. These variations affect properties such as its bandgap.

At present, it is challenging to produce single crystals of SiC with low defect densities; however, high-quality bulk materials can still be obtained by sintering mixtures of polytypes with various proportions and grain sizes – this process being one of the cornerstones of manufacturing SiC and also permitting control over dislocation densities, crystal structures and particle size distribution.

Rhombohedral SiC crystal structures feature an asymmetric lattice with three-dimensional coordination that belongs to the hexagonal family of Bravais lattices. Two-dimensional unit cells feature equal bases with 120deg included angles; however, three-dimensional cell units feature bases radiusing 10angstroms for more robust results.

SiC crystals with rhombohedral symmetry were studied using X-ray single crystal diffraction on a Synergy S Rigaku 4-circles diffractometer equipped with a Cu micro focus source and Eiger 1M Dectric detector, using Crysalis Pro software for data acquisition and processing.

Rhombohedral SiC is widely known for its exceptional physical properties, such as strong elastic response, small interplanar distance and low surface tension. Furthermore, its low melting temperature and excellent thermal conductivity make it ideal for applications involving high temperatures in electronics as well as cutting/grinding applications involving tungsten carbide tools. These characteristics make Rhombohedral-SiC an attractive material to use when combined with Tungsten Carbide tools in cutting applications.

Rhombohedral b-SiC

Silicon carbide has an interwoven crystal structure and exists in different varieties or polytypes. Each form consists of Si and C atoms in equal proportions that are held together with tetrahedral bonds. Each type possesses different electrical properties which depend on its geometry of crystal lattice; for instance, electron transition energy depends on orientation along the crystallographic c-axis – this property is known as anisotropy. Each polytype can also be distinguished by stacking sequences or symmetry;

SiC polytypes most frequently encountered are hexagonal a-form and cubic b-form crystallization patterns. Although other non-cubic variants exist, they are rarely used commercially. A variety of other non-cubic polytypes exist but rarely found commercial products. A-form crystallizes with the wurtzite ABAB stacking pattern and hexagonal symmetry; whereas, for comparison purposes b-form crystallizes using rhombohedral stacking sequence with hexagonal symmetry; its high stability contributes significantly to its exceptional material properties such as large band gap and thermal conductivity.

To create single-crystal SiC, seeds are deposited on a substrate and grown using sublimation growth technology into cylindrical boules, which are then cut into wafers used in semiconductor devices and various applications – typically between 50 mm thick and 30 mm in diameter.

SiC is difficult to grow, yet can still be used to produce larger wafers by slicing off-axis from boules using a mechanical grinder. This approach results in more uniform surfaces while decreasing chances of contamination during slicing processes.

Polytype selection for semiconductor wafer production is of paramount importance. This is because some of their most crucial electrical properties are anisotropic, meaning their values vary along and perpendicular to c-axis axis; this variance can often be explained by differences in seed crystal polytype; 4H-SiC wafers tend to have higher electrical property values due to having stronger C-Si bonds than 6H-SiC crystal.